A wheel alignment device is commonly used to measure, compare with reference data and adjust the characteristic angles of wheels and of the steering system of motor vehicles, particularly camber, toe-in and caster. Correct adjustment of these angles is fundamental for safe travel with low consumption and with low tire wear.
A typical wheel alignment device generally comprises four measurement arms, which are to be engaged with respective wheels and are provided with optical sensors for measuring the relative angles between the wheels. The arms are also provided with an inclinometer to measure the angles relative to the direction of gravity.
The arms are engaged with the wheels or the corresponding rims by means of respective grip elements, which mate with the arms by means of a system constituted by a pivot and a loose hole. The rotation of the arms about the pivots of the grip elements can be prevented or allowed selectively.
The alignment device is further provided with a unit for calculating and representing data, which is connected to the sensors of the arms, in order to receive and process the acquired signals. The alignment device also has an updatable database of the values and tolerances of the characteristic angles for each make and model of motor vehicle.
Moreover, two rotating plates are preferably provided on which the steered wheels are positioned in order to decrease the friction between the wheels and the supporting surface and thus eliminate from the trim tensions which are not natural during travel conditions, particularly when the wheels are steered.
Finally, the system is preferably completed with an auto lift or other flat surface which allows the operator to access below the motor vehicle, for example to adjust its trim.
The steps of a wheel alignment operation are typically five and consist of preparation, runout, measurement, steering and adjustment.
During the preparation step, the motor vehicle is positioned on the auto lift or on another flat surface so that the steered wheels are arranged above the rotating plates, the steering system is straight and the gear is in neutral. The grip elements of the arms are then engaged with the wheels or rims and the arms are connected to the grip elements so as to be perfectly horizontal or, as the jargon says, level.
Since manual fitting of the grip elements is not sufficiently precise for the application, the runout step is typically performed; in this step, first measurements of the sensors of the arms are made at different angles of rotation of the wheels. The processing of these first measurements allows to calculate coefficients (hereinafter “runout errors”) which quantify the misalignment of the axes of the grip elements with respect to the rotation axes of the wheels. This step is described with greater detail hereinafter.
During the measurement step, second measurements are performed by means of the sensors of the arms while the wheels are motionless and straight. The processing of these second measurements, together with the runout errors calculated earlier, allows to determine the characteristic angles of the wheels of the motor vehicle.
During the steering step, the measurement of the sensors of the arms is performed while the wheels are motionless and steered. The processing of these measurements, together with the data acquired earlier, allows to determine the characteristic angles of the steering of the motor vehicle.
Finally, during the adjustment step, the trim of the motor vehicle is adjusted manually and the variations of the characteristic angles are measured and calculated continuously in order to bring such angles within the tolerances provided by the motor vehicle manufacturer.
The factors with which the market assesses a motor vehicle wheel alignment device are measurement speed, measurement precision and cost.
In particular, since the runout step is the most complicated and time-consuming step among the five ones described above, the factors with which the market assesses an alignment device affect the type of runout that is adopted.
For example, the classic method proposed in U.S. Pat. No. 3,892,042 requires approximately one third of the total measurement time.
As mentioned, during the runout step, arm sensor measurements at different angles of rotation of the wheels are performed. Processing these measurements allows to obtain so-called runout errors, i.e., coefficients which quantify the misalignment of the axes of the grip elements with respect to the wheel rotation axes.
If the wheel rotation angles of the wheels are multiples of 90°, it is feasible and sufficiently accurate to position the wheels visually.
Otherwise, accurate angular sensors are needed, because errors in assessing the wheel rotation angles would cause excessive errors in the assessments of the orientations of the rotation axes.
The runout step can be performed on the ground, by moving the vehicle on a surface (hereinafter “rolling runout”), as described in U.S. Pat. No. 5,519,488, or while the vehicle is raised, turning the wheels one at a time (hereinafter “elevated runout”). Rolling runout satisfies market requirements more than elevated runout in terms of speed and precision of measurement, since the wheels are turned simultaneously instead of one at a time; the vehicle does not need to be lifted and lowered; measurements are performed with the suspension settled in functional conditions; measurements after rolling runout are not influenced by the hysteresis of the trim or by effects caused by plays of the suspension due to the lifting and lowering of the vehicle.
In contrast with these advantages, rolling runout has the functional disadvantage of requiring a flat surface which is long enough to make the wheels roll through the runout angles.
Even when using a rolling runout, which requires a single rotation, it is difficult for the lift to be long enough to make the wheels turn through the minimum angle which can be measured visually (90°).
Moreover, even if the bridge were long enough, certainly the rotating plates would not be long enough, consequently affecting the measurement due to the rolling of the wheel on an uneven surface.
In order to obviate these drawbacks, U.S. Pat. No. 4,953,296 and U.S. Pat. No. 5,311,668 have proposed to measure precisely the runout angle, by adding a sensor for measuring the relative angle between the arm and the wheel. However, this approach also requires bearings, in order to leave the arms free to rotate like pendulums about the pivot for connection to the grip elements, and optionally braking systems to prevent the impact of the arms on the ground. Accordingly, cost is penalized thereby.
Alignment devices are known which are based on other operating principles, such as for example the device disclosed in U.S. Pat. No. 6,148,528, but they are generally characterized by a high cost.
One characteristic of known wheel alignment systems which further penalizes these factors is the need to arrange the arms in a horizontal position before each measurement (hereinafter termed “leveling”). Leveling consists in turning the arms about the pivot for connection to the respective grip elements until one of the angles related to gravity, known as level angle, is eliminated with a certain tolerance.
Leveling is required because known calculations performed starting from the measurements of the arms to obtain the characteristic angles of the wheels and of the steering system require measurements with level arms.